In a motor drive device for controlling the driving of a motor, in order to set the rotation speed of a motor to a desired value, a drive voltage applied to one end of a motor coil (and thus a drive current flowing through the motor coil) needs to be controlled to be a desired value.
Accordingly, conventional motor drive devices have commonly adopted a configuration in which, as shown in FIG. 10, bipolar transistors Tr1 to Tr4 are used as switch elements in an H-bridge circuit, and means adapted to control base currents of upper-side transistors Tr1 and Tr2 according to a control voltage Vref externally fed as a drive voltage setting signal is provided (transistors Tra and Trb and constant current sources Ia and Ib), to thereby control a drive voltage applied to one end of a motor coil L (and thus a drive current flowing through the motor coil L) to be a desired value (so-called dropper-type regulator configuration). A control circuit CNT in this figure is means adapted to control the on-off state of the transistors Tr1 to Tr4 (i.e., the operation mode of the motor) according to the logics of externally fed operation mode control signals FIN and RIN (binary signals).
In the motor drive device configured as described above, while the upper-side transistor Tr1 (Tr2) is kept on, a drive voltage (a voltage approximately as high as the control voltage Vref) obtained by raising the control voltage Vref only by the value of 1Vf of the transistor Tra (Trb) and then dropping the control voltage Vref only by the value of 1Vf of the transistor Tr1 (Tr2) is applied to the one end of the motor coil L.
As other conventional arts related to the present invention, there have been disclosed and proposed: a motor drive control system in which field effect transistors are used as switch elements in an H-bridge circuit and gate voltages of the transistors are controlled by pulse width modulation control (hereinafter referred to as PWM (pulse width modulation) control) (see Patent Publication 1); and a technology in which, with respect to a triangular wave generating circuit that outputs the charge-discharge voltage of a capacitor as a triangular wave, the frequency of the outputted triangular wave is maintained to a predetermined frequency by such control that the amplitude of the triangular wave is reduced with reduction of the power source voltage, and further, the value of a current for charging and discharging the capacitor is reduced according to this amplitude (see Patent Publication 2 filed by the applicant of the present application).
Patent Publication 1 JP-B-3665565
Patent Publication 2 JP-A-2002-223563